It is conventional to incorporate an absorbing compound or absorber, in particular, an ultraviolet ray absorber, in an image element to absorb light in a specific wavelength region. The ultraviolet ray absorbing light-insensitive layer is used, for example, in silver halide photographic elements, to control the spectral composition of light incident upon a photographic emulsion layer, and to absorb or to remove ultraviolet light produced by static discharge, which occurs when the surfaces of the photographic element come into contact during production or treatment processes. Electric charges are generated by friction of separation. When accumulation of static electricity by charging reaches a certain limiting value, atmospheric discharge occurs at a particular moment and a discharge spark fires at the same time. When the photographic element is exposed to light by discharging, static marks appear after development.
Different methods for incorporating an ultraviolet ray absorber into a photographic element have been described in, for example, US Pat. Nos. 2,739,888, 3,215,530, 3,352,681, and 3,707,375, where an oil soluble absorber is dissolved in a high boiling organic solvent, and mixed under high shear or turbulence together with an aqueous medium, which may also contain a surfactant, in order to break the organic phase into submicron particles dispersed in the continuous aqueous phase. However, when such dispersions are used in a light-insensitive layer, the layer becomes soft and the mechanical properties of the layer is lowered. Furthermore, even if no high boiling solvent is used, many absorbers themselves are liquid, and they therefore can have a detrimental effect on the mechanical properties of the layer and adhesion with the adjacent layer. In order to prevent these problems, a large amount of gelatin has been used in the layer containing ultraviolet ray absorbers.
When the above dispersions containing the ultraviolet ray absorbers are incorporated in the outermost protective layer of a photographic element, both the organic solvent and ultraviolet ray absorber often migrate to the surface of the layer. These leached out oily materials not only degrade the physical properties of photographic elements, but also can easily be transferred to manufacturing and processing equipment, causing transportation difficulties. Furthermore, the outermost protective layer often comprises a polymeric matting agent such as poly(methyl methacrylate) or polystyrene. The coupler solvents used for dissolving ultraviolet ray absorbers are good solvents for these polymers. When a polymeric matting agent and coupler solvents are contained in the same solution, coupler solvents partition into, swell, and deform the polymeric matte particles. This greatly diminishes surface matting effect, causing unexpected manufacturing difficulties. Therefore, an additional overcoat layer has to be used to minimize the undesirable blooming phenomenon and the interactions between coupler solvents and polymeric matting agents, which often requires an additional coating station, thereby increasing manufacturing cost.
U.S. Pat. No 5,110,717 describes a method of incorporating an ultraviolet ray absorber or coupler by mechanically grinding a crystalline material to a desired particle size in a liquid that is not a solvent for the material, heating the crystalline particles to above their melting point, and cooling the melted particles to form amorphous particles.
Ultraviolet ray absorbing polymer particles obtained by polymerization of ultraviolet ray absorbing monomers are also known in the art. Different methods can be used to prepare such particles, for example, by emulsion polymerization, or by microemulsion polymerization, or by dispersion of preformed ultraviolet ray absorbing polymers. Disadvantages of using such polymer particles are their high cost and poor light stability.
Another method of incorporating an ultraviolet ray absorber into a photographic element is by loading such an absorber into pre-formed latex particles as described in U.S. Pat. Nos. 4,199,363, 4,304,769, 4,247,627, and 4,368,258. In this process, a hydrophobe is first dissolved in a water miscible organic solvent and then blended with an aqueous latex. Removing the water miscible solvent subsequent to loading requires large scale processing equipment and lengthy processing times, which increases the expenses of the incorporation procedure and cost of the resulting products.
U.S. Pat. No. 5,536,628 describes a process for incorporating absorbing compounds into a pre-formed latex polymer particle. In the process a polymer latex of known solids is heated with stirring to 70 to 80.degree. C. The absorber is heated until it reaches its liquid state and is mixed with the polymer latex at high shear to generate an emulsion. The emulsion is then passed through a high energy homogenizer at least once to form an absorbing compound impregnated latex polymer dispersion.
Processes described in U.S. Pat. Nos. 5,536,628, 4,199,363, 4,304,769, 4,247,627, and 4,368,258 can still result in residual ultraviolet ray absorbers in the aqueous phase, which can then crystallize or form large oil droplets during storage and use. The ultraviolet ray absorbers can also diffuse out of the polymer particles to aggravate crystallization and oil droplet formation further shortening the shelf life of ultraviolet ray absorbing polymer particles. In addition, even small amounts of oil droplets or crystals can significantly degrade the physical and photographic properties of a photographic element by generating spot defects in the final coated layers.